Symmetry-Shaped Singularities in High-Temperature Superconductor H₃S

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Chemistry

Background:

• H3S exhibits one of the highest measured superconducting critical temperatures (203 K).
• This phenomenon is linked to a singularity in the electronic density-of-states, characterized by a flat-band region with saddle points at the Fermi level.
• This electronic structure results in a giant electron-phonon coupling constant, crucial for superconductivity.

Purpose of the Study:

• To elucidate the roles of atomic orbital interactions and space group symmetry in shaping the electronic singularity in H3S.
• To understand the origin of the flat-band region and saddle points at the Fermi level.
• To explain the weak pressure dependence of the superconducting critical temperature in H3S.

Main Methods:

• Theoretical investigation using first-principles calculations.
• Analysis of electronic band structure within the body-centered cubic Brillouin Zone.
• Examination of orbital mixing and symmetry-enforced energy inversions.

Main Results:

• Atomic orbital interactions and space group symmetry cooperatively form the electronic singularity.
• The body-centered cubic Brillouin Zone's unique 2D hypersurface (connecting Γ, H, and N points) is critical.
• Symmetry-enforced s-p energy inversion between Γ and H points causes the collapse of saddle point lines, creating nonbonding states.

Conclusions:

• The electronic singularity and resulting giant electron-phonon coupling in H3S are governed by the interplay of orbital interactions and crystal symmetry.
• The nonbonding nature of saddle-point states explains the superconductor's insensitivity to pressure.
• The theoretical framework demonstrates the potential for engineering flat bands and singularities in 3D lattices via symmetry considerations, applicable to designing novel superconductors.